Digital printing control using a spectrophotometer

ABSTRACT

What is disclosed is a novel system and method for xerographic Dmax control based upon measurements made on the printed paper using an inline spectrophotometer (ILS) or similar device. The disclosed method is based upon directly measuring the color to actuator sensitivity. Each of the separations is controlled independently using an actuator specific to that color separation. The present method is effective at controlling the color of the solid primaries. The fact that the vector of change is highly correlated with solid color variation seen in the field suggests that the teachings hereof effectively increase the solid color stability. Increased solid color stability increases the color stability throughout the printer gamut and the stability of the gamut boundaries, which increases the robustness of gamut mapping algorithms. Advantageously, the present method can be combined with existing ILS-based maintenance architectures.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is related to commonly assigned and co-pendingU.S. patent application Ser. No. 12/839,620, (Docket No.20100103-US-NP), entitled: “Digital Printing Control Using ASpectrophotometer, filed Jul. 7, 2010, which is incorporated herein inits entirety by reference.

TECHNICAL FIELD

The present invention is directed to Inline Spectrophotometer (ILS)xerographic Dmax control system and methods which utilize a fleetgradient (universal or ‘fixed’ sensitivity) to control each colorseparation of an N-color marking device having a single xerographicactuator specific to each color separation.

BACKGROUND

In current production printers, the solid color of each separation(primary) is controlled based on the achievement of a nominal DMA(developed mass per unit area) as measured on the photoreceptor (usuallya belt or drum). Typically, reflectance measurements are made using anoptical sensor, e.g., an ETAC (enhanced toner area coverage)densitometer, to infer DMA via a model relating the two, i.e., mass as afunction of reflectance and a target DMA is determined by substitutingthe nominal DMA into this model. Then, while printing, measuring thereflectance of patches in the inter-document zone, xerographic processcontrol loops adjust the xerographic actuators such that the reflectancetracks the target value. Examples of common xerographic actuatorsinclude ROS (raster output scanner) exposure, the photoreceptor voltages(charged and/or discharged voltage), donor and/or magnetic rollvoltages, and toner concentration. Although functional, this type ofprocess control system, presently used on iGen3 and iGen4 systems, issubject to color variation due to variation in the environment, media,sensors, xerographic processes, operating conditions, downstream effectsfrom transfer & fusing, etc.

Accordingly, what is needed in this art are increasingly sophisticatedsystems and methods which utilize a fleet gradient for xerographic Dmaxcontrol to increase color stability and gamut mapping robustness in anN-color marking device having a single xerographic actuator specific toeach color separation.

INCORPORATED REFERENCES

The following U.S. Patents, U.S. Patent Applications, and Publicationsare incorporated herein in their entirety by reference.

“Systems And Methods For Printing Images Outside A Normal Color Gamut InImage Forming Devices”, U.S. Publication No. 20060227395, to Mestha etal.

“Controlling Process Color In A Color Adjustment System”, U.S. patentapplication Ser. No. 12/536,600, to Mestha et al. (Attorney Docket No.20090416-US-NP), filed Aug. 6, 2009.

BRIEF SUMMARY

What is disclosed is a novel Inline Spectrophotometer (ILS) xerographicDmax control algorithm which is based upon a fleet gradient (universalsensitivity). The intention hereof is to provide an algorithm tocalibrate a print engine's color at the solid area level and regulate itduring runtime. This is particularly important since digital areacoverage fundamentally cannot compensate for a solid too light conditionand it has been experimentally verified that the color response todigital area coverage can be significantly different than that of mass(at least in the range of ˜60% to 100%). The response to area coverageis almost orthogonal to mass for Cyan and Magenta. Black also has somedifference in effects. Yellow has very little. The difference betweenarea coverage and mass effects is primarily due to unwanted absorptions,which Cyan and Magenta are known to have. The method disclosed is basedon a fleet gradient (fixed sensitivity) wherein each of the separationsof an n-separation color marking device is controlled independentlyusing an actuator specific to that separation. The present methodassumes a known gradient between an actuator and the L*a*b* response.From this, the current nominal point is measured and, given the errorfrom target, an adjustment or offset can be made to the actuatorsetting. After making the adjustment, if the difference between the newpoint (in L*a*b*) and predicted value is larger than an establishedthreshold, a second adjustment can be made from the original point usinga gradient based upon fitting a line between the original point and thenew point. The present method is effective at controlling the color ofthe solid primaries. The fact that the vector of change is highlycorrelated with solid color variation seen in the field suggests thatthe teachings hereof effectively increase the solid color stability.Increased solid color stability increases the color stability throughoutthe printer gamut and the stability of the gamut boundaries, whichincreases the robustness of gamut mapping algorithms. Advantageously,the present method can be combined with existing ILS-based maintenancearchitectures.

In one example embodiment, the present method for calibration of amarking device's output color as measured at the solid area levelinvolves performing the following. First, a color separation of ann-separation marking device is selected for testing. The marking devicehas at least one adjustable actuator which regulates an amount ofcolorant deposited on a substrate at the solid area level and theactuator has been previously set to a predetermined first operatingpoint. Next, a target color parameter vector T is selected or otherwiseidentified at the solid area level. The target color parameter comprisesa point in color parameter space. A solid area patch of the target colorparameter vector T is printed on a substrate using the marking devicewith the actuator set to the first operating point. The printed patch isthen measured in order to obtain a nominal measured color parametervector N. The amount of offset is determined based upon the target colorparameter vector T, the nominal measured color parameter vector N, and afleet gradient {right arrow over (G)}_(F). Various embodiments for thisoffset are described herein further. The determined amount of offset canthen be applied to the actuator setting such that the actuator isadjusted to a second operating point. Various embodiments are disclosed.

Many features and advantages of the above-described method will becomereadily apparent from the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the subject matterdisclosed herein will be made apparent from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram illustrating one embodiment of the presentmethod using a mass-based xerographic process control loop (duringruntime);

FIG. 2 is a flow diagram of one example embodiment of the present methodfor determining an optimal actuator setting based upon a fleet gradient(fixed sensitivity);

FIG. 3 is an illustration which helps explain various aspects ofdetermining an actuator setting as described with respect to the flowdiagram of FIG. 2;

FIG. 4 is an illustration, similar to the color parameter space of FIG.3, with the addition of a measured color parameter 407(M);

FIG. 5 shows functional block diagram of a color marking device arrayedin a networked configuration capable of color reproduction inn-separations; and

FIG. 6 is a block diagram of one example embodiment of a special purposecomputer useful for implementing one or more aspects of the presentmethod, as shown and discussed with respect to the above-describedillustrations.

DETAILED DESCRIPTION

What is disclosed is novel system and method for in-linespectrophotometer (ILS) xerographic Dmax control algorithm forcalibration of a n-separation marking device's output color as measuredon a media substrate at the solid area level. The fact that the vectorof change is highly correlated with solid color variation suggests thatthe teachings hereof increase the solid color stability which increasesthe color stability throughout the printer gamut and the stability ofthe gamut boundaries. This, in turn, increases the robustness of gamutmapping methods employed. Further, if combined with existing ILS-basedmaintenance routines, additional measurements (prints) are not required.

It should be understood that one of ordinary skill in this art would bereadily familiar with many facets of color science and image processingand other techniques and algorithms commonly found in the color scienceand xerographic arts. Those of ordinary skill would be familiar with thetext: “Digital Color Imaging Handbook”, 1st Ed., CRC Press (2003),ISBN-13: 97808-4930-9007, and “Control of Color Imaging Systems:Analysis and Design”, CRC Press (2009), ISBN-13: 97808-4933-7468, bothof which are incorporated herein in their entirety by reference.

Non-Limiting Definitions

A “color parameter space” is any standard color space that is commonlyused to define or describe color, e.g. CIE XYZ, CIE L*a*b*, CIE L*u*v*,sRGB etc.

A “device-dependent color space” is a color space that is non-standardand cannot be used to commonly define colors without additionalinformation such as the characteristics of the rendering device.

A “single separation color” refers to a color specified with only one ofthe color separations for a target marking system. For example, for aCMYK printer, a C-only test patch is a single-separation color patch.

A “multi-separation color” refers to a color specified with more thanone of the color separations for a target marking system. For example,for a CMYK printer, a red test color is a multi-separation color testpatch, using combinations of M and Y separations at some pre-determinedlevels.

A “n-separation marking device” is color marking device or system thatis able to generate a color image in multi-separation color using imagedata or data generated from image data.

A “colorant' refers to the medium used for rendering a particular colorseparation which, in forming a multi-colored image, is combined with oneor more other colorants to achieve image colors throughout the spectrum.Each color separation thus may have its own corresponding colorant.

A “substrate” is a surface upon which toner is deposited using a processthat involves one or more actuators. Such surfaces may also take theform of a photoreceptor belt or drum on which charged toner particlesare deposited to form an image. Toner particles are chargedtriboelectrically, in either a single component development process or atwo-component development process.

A “media substrate” is a substrate, such as paper or transparency film,that is used as a final output surface for an image.

An “actuator” is a controller or device element such as, for example, acorotron or scorotron wire voltage or a scorotron grid voltage ROSpower, or development bias voltage, which is adjustable such that ameasurement received from an ESV (electrostatic voltmeter), ETAC,densitometer, colorimeter, or spectrophotometer, is driven toward avoltage target value or set point. Adjusting an actuator setting orotherwise changing the actuator's operating point, darkens or lightensan image by controlling the amount of toner deposited on the surface ofa media substrate.

A “reflectance sensing device”, is to a device capable of measuring anamount of light reflected from a sample. A reflectance sensing devicecan be any of: a full width sensing array, a spectrophotometer, acolorimeter, or a densitometer.

A “spectrophotometer” is a reflectance sensing device which measures thereflectance over many wavelengths and provides distinct electricalsignals corresponding to the different levels of reflected lightreceived from the respective different illumination wavelength rangesusing multiple channels.

A “colorimeter” is a reflectance sensing device which typically hasthree illumination channels and which provides output color values inthe trichromatic quantity known as RGB, (red, green, blue) as read by asensor receiving reflected light from a surface.

A “densitometer” is another reflectance sensing device which typicallyonly has a single channel and simply measures the amplitude of lightreflectivity from the test surface, such as a developed toner test patchon a photoreceptor, at a selected angle over a range of wavelengths,which may be wide or narrow. The output of the densitometer is theoptical density of the test sample.

A “target color parameter vector” defines a point of desired color in acolor parameter space by a set of coordinates. If the color parameterspace is the L*a*b* space, for example, a target color x_(c) has a colorparameter vector with coordinates {L*_(c), a*_(c), b*_(c)}.

A “nominal color parameter vector” defines a color in a color parameterspace obtained from measuring a solid area patch printed on a substrateusing the color marking device with an actuator for this colorseparation set to a nominal operating point.

A “measured color parameter vector” defines a color in color parameterspace obtained from measuring, using a reflectance sensing device, asolid area patch printed using an actuator operating point for thiscolor separation.

“Solid area lever” describes a setting or an amount of area coverage toa maximum reasonable density, given the properties of the printerhardware under expected conditions. Printing at the solid area levelinvolves transferring a continuous amount of colorant to all locationswithin that area of the substrate. In a digital printer that useshalftoning, for instance, the solid area level is 100% area coverage,where all the halftone dots are completely filled in. In the halftoneworld, the ‘solid area’ is synonymous with 100% area coverage.

To “converge the xerographics” means to allow a xerographic device toadjust to changes made by one or more actuators in response to thedeterminations made herein.

A “xerographic device” refers to any system or device capable ofreceiving a signal of a color image and reducing that signal to aviewable form.

General Discussion

What is presented herein is a ILS xerographic Dmax control method. Themaximum printable density (Dmax) and resulting solid color of productionprinters (e.g., iGen3/iGen4) is currently setup using on-belt(photoreceptor) mass measurements for each separation rather thanon-paper color measurements. Xerographic process control loops operateon each separation to maintain the Dmax on the photoreceptor at nominaltargets, using xerographic actuators like ROS exposure and thedevelopment field to control each separation. However, as discussed inthe background section, due to variation in the environment, media,sensors, xerographic processes, operating conditions, downstream effectsfrom transfer & fusing, and the like, the color of a particular machinecan have large variation throughout its gamut and even in the shadowregions regardless of how well the mass of individual separations iscontrolled on the photoreceptor. Such color variation can lead to colorstability issues and may compromise the effectiveness of gamut mapping.Directly measuring the color increases color stability and gamut mappingrobustness when compared with on-belt measurement methods. Measurementsmade on the photoreceptor using an ETAC or similar device can beoptimized based upon measurements made on the printed paper using aninline spectrophotometer (ILS) or similar device. As with currentmass-based methods, the present method comprises a setup routine thatcan be performed while the device is offline. To stabilize the colorduring runtime, a current mass-based approach (FIG. 1) is employed.Control loops operate on each color separation to maintain the referencevalue established by the ILS xerographic Dmax control algorithm.

The basic fleet gradient approach assumes if the fleet gradient can beconsidered known and stable, it can be combined with a nominal point tocreate a linear model without requiring additional direct measurements.The embodiments hereof involve subsequent steps of performance checkingcolor measurements. If the measured color of each separation is notwithin specified thresholds, the approaches are repeated. Specifiedthreshold(s) may involve conditions such as:

-   -   the current color is within a threshold distance of the last        predicted color (good model)    -   the current color is within a threshold distance of the next        predicted color (small next step)    -   the current color is within a threshold distance of the target        color (good primary color).

If the color of each separation is not within a specified threshold(s),a new gradient ({right arrow over (G)}₂) can be determined from theoriginal (nominal) color and the adjusted (measured) color as measuredby the ILS. Determining the new gradient is discussed herein furtherwith respect to FIG. 4 and Eq. 7. The new gradient (of Eq. 7) is thenused to produce a new actuator offset value, as discussed herein furtherwith respect to Eq. 8.

Example Flow Diagram

Reference is now being made to the flow diagram of FIG. 2 whichillustrates one example embodiment of the present method for determiningan actuator setting based upon a fleet gradient (fixed sensitivity).This embodiment assumes that a fleet gradient {right arrow over (G)}_(F)has already been determined. Processing begins at 200 and immediatelyproceeds to step 202. Various aspects of this embodiment are discussedin conjunction with L*a*b* color space 300 of FIG. 3.

At step 202, a first color separation of an n-separation marking deviceis selected for processing. The marking device has at least oneadjustable actuator per color separation which regulates an amount ofcolorant deposited on a substrate at the solid area level. The one ormore actuators operable for this color separation have been previouslyset to a nominal operating point. Such an operating point can readily bedetermined by one of ordinary skill.

At step 204, a target color parameter vector T is selected at the solidarea level. The target color parameter vector comprises a point in colorparameter space. One example target color parameter vector T is shown at302(T). It should be appreciated that each of the color points of FIG. 3comprises a vector which extends from the origin of L*a*b* color space300.

At step 206, a solid area patch containing the target color is printedon a substrate using the marking device and the printed patch ismeasured using a reflectance sensing device to obtain a nominal measuredcolor parameter vector N. One such reflectance sensing device is anin-line spectrophotometer (ILS). One example nominal color parametervector N is shown at 305(N). The measurement readings (and controlsdetermined thereby) can be in-line, off-line, or manually obtainedusing, for example, a hand-held reflectance sensing device.

At step 208, a determination is made whether all primary solids are in adesired range. This determination is made using a metric for calculatingan amount of a color difference between the nominal measured colorparameter vector N and the target color parameter vector T, and thevector of the difference with respect to the fleet gradient may also beconsidered as part of the determination. This color difference (ordistance), at 309, can be Euclidean, deltaE2000, or any other equivalentcolor distance metric CIECAM02. Generally, a Euclidean distance betweentwo colors in, for example, L*a*b* space, having coordinates expressedin terms of {L*, a*, b*}, is given by:

ΔE* _(ab)=√{square root over ((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over((ΔL*)²+(Δa*)²+(Δb*)²)}{square root over ((ΔL*)²+(Δa*)²+(Δb*)²)}.   (1)

In a cylindrical coordinate representation of L*a*b* space, theEuclidean distance between two colors x₁ and x₂ having coordinates {L*₁,h*₁, C*₁} and {L*₂, h*₂, C*₂}, respectively, is given by:

ΔE* _(ab)=√{square root over ((ΔL*)² +C* ₁ +C* ₂−2C* ₁ C* ₂ cos(h* ₂ −h*₂))}{square root over ((ΔL*)² +C* ₁ +C* ₂−2C* ₁ C* ₂ cos(h* ₂ −h* ₂))}.  (2)

If, at step 208, all the primary solids are in the desired range, thenprocessing continues with respect to step 214 wherein a determination ismade whether more color separations in this n-separation device remainto be processed. If, at step 208, all the primary solids are not withindesired ranges, then processing continues with respect to step 210.

At step 210, calculate a new actuator level and set the device to a newactuator setting. One example fleet gradient {square root over (G)}_(F)is shown at 304(G_(F)). Intersection point 310(B) is a best color.Vector 311(T−B) from target color parameter vector 302(T) is orthogonalto a line starting at nominal color parameter vector 305(N) andextending in the direction of 304(G_(F)). Line 311(T−B) is orthogonal tothe line intersecting best point 310(B).

The following equations are used to derive the actuator offset value x.

B=N+x{right arrow over (G)} _(F)   (3)

where B corresponds to 310(B), N corresponds to 305(N), {right arrowover (G)}_(F) is the fleet gradient 304(G_(F)), and {right arrow over(G)}_(F) is 312(x {right arrow over (G)}_(F)), as shown in FIG. 3.

{right arrow over (G)} _(F)•(T−B)=0   (4)

where T corresponds to 302(T), the vector (T−B) corresponds to 311(T−B),and the symbol ‘•’ is the dot product operation. It should beappreciated that 311(T−B) is orthogonal to a line (at 303) extendingfrom 305(N) through 310(B) in the direction of 304({right arrow over(G)}_(F)).

A substitution of Eq. 3 into Eq. 4, produces the following:

{right arrow over (G)} _(F)•(T−N−x {right arrow over (G)} _(F))=0.   (5)

A manipulation of Eq. 5 produces the relationship defining the actuatoroffset which will produce best color 310(B), as given by:

$\begin{matrix}{x = {\frac{{\overset{->}{G}}_{F \cdot} \cdot \left( {T - N} \right)}{\left( {{\overset{->}{G}}_{F} \cdot {\overset{->}{G}}_{F}} \right)}.}} & (6)\end{matrix}$

At step 212, converge the xerographics to the new actuator setting.

At step 214, a determination is made whether any more color separationsfor this n-separation color marking device are intended to be processed.If so, then processing repeats with respect step 202 wherein a nextcolor separation is selected for processing. Processing repeats in sucha manner until all color separations for the color marking device havebeen processed accordingly. Thereafter, processing stops.

It should be understood that the flow diagrams are illustrative. One ormore of the operative steps may be performed in a different order. Otheroperations, for example, may be added, or consolidated. Variationsthereof are intended to fall within the scope of the appended claims.All or portions of the flow diagrams may be implemented partially orfully in hardware in conjunction with machine executable instructions incommunication with various components of such a system.

Performance Checking and New Gradient Determination

The following is given with reference to the color parameter space 400of FIG. 4 which is similar to the color parameter space of FIG. 3 withthe addition of a measured color point. Upon having set the actuator toa new (second) operating level using Eq. 6, a solid area patch of thetarget color parameter 402(T) can then be printed on a substrate usingthe marking device and a measured color parameter at 407(M), obtainedfrom ILS measurements of the printed patch. As in FIG. 3, the nominalcolor parameter is shown at 405(N), the best color point is at 410(B),line 411(T−B) is orthogonal to a line 403 extending from 405(N) andpassing through 410(B) and 407(M) in the direction of 404({right arrowover (G)}₂), and 412(x₂{right arrow over (G)}₂) is a vector.

The measured color value 407(M) can then be compared to the target color402(T) to determine whether the printed patch is within predeterminedthreshold(s). If not then a new gradient can be determined and theprocess iterated. A determination of the new gradient follows Eqs. 3-6,as discussed above. This new gradient is given by:

$\begin{matrix}{{\overset{->}{G}}_{2} = \frac{\left( {M - N} \right)}{z}} & (7)\end{matrix}$

where M corresponds to 407(M), N corresponds to 402(N), and z is adifference between the first and second operating points.

Using this new gradient, a new actuator offset y can be defined asfollows:

$\begin{matrix}{y = \frac{{\overset{->}{G}}_{2} \cdot \left( {T - M} \right)}{\left( {{\overset{->}{G}}_{2} \cdot {\overset{->}{G}}_{2}} \right)}} & (8)\end{matrix}$

where T corresponds to 402(T), M corresponds to 407(M), and {right arrowover (G)}₂ is the new gradient 404({right arrow over (G)}₂).

The new actuator offset is then used to make another adjust to theactuator settings of the marking device to obtain a third operatingpoint. Using the third operating point, measurements can again be takenand the above-described steps repeated, as desired, until the measuredcolor values are within acceptable levels as defined by thepredetermined threshold(s).

Example Functional Block Diagram

Reference is now being made to the system of FIG. 5 which shows afunctional block diagram of a color marking device arrayed in anetworked configuration capable of color reproduction in n-separations,wherein various aspects of the present method as shown and describedwith respect to the flow diagram of FIG. 2 find their intendedimplementations.

In the illustrated system of FIG. 5, a plurality of color markingengines 530 and 532 are incorporated into system 514 shown connected tonetwork 535 via communication bus 534. Each of the marking engines areselectable for printing image data received from computer system 510 orfrom a remote device, shown generally at 512. One or more of theplurality of marking engines may be associated with separate printersystems. Although only 2 marking engines are illustrated, it should beappreciated that the illustrated system has n-marking engines where n>2.

Marking devices 530, 532 comprise any suitable device for applyingimages to print media, such as xerographic marking devices, inkjetmarking devices, or the like. The marking device includes many of thehardware elements employed in the creation of desired images byelectro-photographical processes. The marking devices are shown having acharge retentive surface, such as a rotating photoreceptor in the formof a belt or drum, either at 580. The images are created on a surface ofthe photoreceptor. Disposed at various points around the circumferenceof the photoreceptor are xerographic subsystems which include a cleaningdevice, a charging station for each of the colors to be applied (one inthe case of a monochrome marking device, four in the case of a CMYKmarking device), such as a charging corotron, an exposure station, whichforms a latent image on the photoreceptor, a developer unit, associatedwith each charging station for developing the latent image formed on thesurface of the photoreceptor by applying a toner to obtain a tonerimage, a transferring unit, such as a transfer corotron, transfers thetoner image thus formed to the surface of a print media substrate, suchas a sheet of paper, and a fuser 582, which fuses the image to thesheet. The fuser generally applies at least one of heat and pressure tothe sheet to physically attach the toner and optionally to provide alevel of gloss to the printed media. While both illustrated markingengines 530, 532 may be similarly configured, it is also contemplatedthat the making devices may differ in one or more respects. The markingengines include on-belt sensor 586. Marking engines 530 and 532 caninclude any device for rendering an image on print media. The markingdevices may be single colorant (monochrome, e.g., black) marking devicesor multiple colorant (color) marking devices, such as CMYK devices. Theimage rendering device 514 incorporating the plurality of markingengines can be a copier, laser printer, bookmaking machine, facsimilemachine, or a multifunction machine. An image to be printed generallyincludes information in electronic form which is to be rendered on theprint media by the image forming device and may include text, graphics,pictures, and the like. The operation of applying images to print media,for example, graphics, text, photographs, etc., is generally referred toherein as printing or marking. As is known in this art, the operation ofprinting involves mapping colorant values to device-dependent colorantvalues.

Spectrophotometer 516 captures image data from example test patch 568and provides color image measurements to ILS Dmax Control System 560either directly or via an intermediate processing component. On-papersensor 516 includes a light source 572, such as an LED bar or otherlight source which directs light to onto the image 568 and furtherincludes a detector 574 which detects reflected light. Spectrophotometer516 includes one or two substantially linear elongated arrays of closelyspaced multiple LED illumination sources transversely spanning the paperpath and which are sequentially illuminated to illuminate a transverseband across a printed sheet moving in the paper path, and acorresponding array of multiple closely spaced different color sensitivephoto-detectors, which are positioned to detect and analyze lightreflected from the sheet. Sensor 516 may further be configured tomeasure optical density. In addition to receiving information fromsensor 516, color image measurements are received by control system 560from on-belt sensor 586. Virtual sensors may be further be used tosupplement the information obtained by sensor 516 or on-belt sensor 586.In one embodiment, on-belt sensor 586 and on-paper sensor 516 comprise afull width array (FWA) which generates digitized image data for eachscan line. It should be appreciated that the on-belt sensor 586 and/oron-paper sensor 516 can be an in-line sensor, such as one which islocated within the printer for example, in an output tray, or,alternatively, may be a stand-alone device or incorporated into, forexample, a dedicated scanner and diagnostic system. Each marking devicemay be associated with its own dedicated sensor. While the sensor 516may detect values of multiple colorants it may detect only a singlecolorant or may detect luminosity of the image. The sensor may include agloss meter which measures the specular reflection of the image. On-beltsensor 586 and/or on-paper sensor 516 may transmit data after completingall the measurements from multiple sheets. Alternatively, the on-beltand/or on-paper sensor transmits data during or after the marking ofeach sheet either via a wired or wireless connection.

ILS Dmax Control System 560 is in communication with sensor 516 and eachof the plurality of marking engines 530, 532 and any of the componentscontained therein. Control System 560 is also in communication withworkstation 510 via communication bus 534 and also remote device 512 vianetwork 535. ILS Dmax Control System performs the present method, asdescribed above. The control system includes a gradient processor 576, amemory 578 and a processor 564. Memory 578 is intended to represent anytype of machine readable medium such as RAM, ROM, magnetic disk or tape,optical disk, flash, holographic, and the like. In one embodiment,memory 578 comprises a combination of RAM and ROM. In variousembodiments, actuator control system 560 is a special purpose computersystem as discussed herein further with respect to the special purposecomputer system of FIG. 6. In the illustrated embodiment, the gradientprocessor 576 obtains receives color measurements and target colors andcalculates an optimal actuator operating point, as described above invarious embodiments, which minimizes a distance between the predictedprinted color parameter vector and the target color parameter vector.

Control system 560 may further convert received color image signals intoa suitable form for rendering and may further functionality formonitoring a performance of the marking devices. The control system mayinclude a print model generator which processes information receiveddirectly or indirectly from device 530, 532, including acquired imagedata from sensor 516. Control system 560 may also utilize informationfor real-time modifying of colors to be printed of jobs submitted to themarking engines, or this functionality may be performed by a separateprocessing component. Control system 560 also includes a test imagegenerator which generates image data for a test pattern. The testpattern may include one or more color test patches.

ILS Dmax Control System 560 may further incorporate special hardwareand/or software for performing commonly known image processingtechniques such as, for example, Fourier transform analysis,histogramming, defect detection and the like. A calibration circuit mayfurther be include for processing at least some of defects introduced bysensor 516. Such a circuit would determine the image quality parametersand/or detect the presence of image quality defects and makeadjustments. Such defects and parameters include, but are not limitedto, lines, bands, streaks, mottle, and the like. Control system 560 maybe placed in communication with system capable of performing AutomatedImage Quality Diagnostics (AIQD) techniques to aid in diagnostics. Suchdiagnostic tools collect and analyze data from the image itself and alsofrom the printer's internal diagnostic systems. Using a combination ofquantitative analysis and qualitative reasoning, the AIQD program thenobtains a diagnosis and recommendation. Other image defect evaluationtechniques may also be utilized for generating information such as, forexample, tone reproduction curve correction models used to compensatefor static defects.

It should be appreciated that any of the modules and processing unitsshown and described with respect to the block diagram of FIG. 5, andsome or all of the functionality described for any of these modules maybe performed, in whole or in part, within workstation 510 or by aspecial purpose computer system. It should be appreciated that variousmodules may designate one or more components which may, in turn, eachcomprise software and/or hardware designed to perform a specificfunction. A plurality of modules may collectively perform a singlefunction. A module may have a specialized processor capable of readingmachine executable program instructions. A module may comprise a singlepiece of hardware such as an ASIC, electronic circuit, or specialpurpose processor. A plurality of modules may be executed by either asingle special purpose computer system or a plurality of special purposecomputer systems in parallel. Connections between modules include bothphysical and logical connections. Modules may further include one ormore software/hardware modules which may further comprise an operatingsystem, drivers, device controllers, and other apparatuses some or allof which may be connected via a network. It is also contemplated thatone or more aspects of the present method may be implemented on adedicated computer system or workstation, and may also be practiced indistributed computing environments where tasks are performed by remoteprocessing devices that are linked through a communication network. In adistributed computing environment, program modules for performingvarious aspects of the present system and method. Other embodimentsinclude a special purpose computer designed to perform the methodsdisclosed herein.

Example Special Purpose Computer System

Reference is now being made to FIG. 6 which illustrates a block diagramof one example embodiment of a special purpose computer system forimplementing one or more aspects of the present method as described withrespect to the embodiments of the flow diagrams hereof and the blockdiagram of FIG. 4. Such a special purpose processor is capable ofexecuting machine executable program instructions. The special purposeprocessor may comprise any of a micro-processor or micro-controller, anASIC, an electronic circuit, or special purpose computer. Such acomputer can be integrated, in whole or in part, with a xerographicsystem or a color management or image processing system, which includesa processor capable of executing machine readable program instructionsfor carrying out one or more aspects of the present method.

Special purpose computer system 600 includes processor 606 for executingmachine executable program instructions for carrying out all or some ofthe present method. The processor is in communication with bus 602. Thesystem includes main memory 604 for storing machine readableinstructions. Main memory may comprise random access memory (RAM) tosupport reprogramming and flexible data storage. Buffer 666 stores dataaddressable by the processor. Program memory 664 stores machine readableinstructions for performing the present method. A display interface 608forwards data from bus 602 to display 610. Secondary memory 612 includesa hard disk 614 and storage device 616 capable of reading/writing toremovable storage unit 618, such as a floppy disk, magnetic tape,optical disk, etc. Secondary memory 612 may further include othermechanisms for allowing programs and/or machine executable instructionsto be loaded onto the processor. Such mechanisms may include, forexample, a storage unit 622 adapted to exchange data through interface620 which enables the transfer of software and data. The system includesa communications interface 624 which acts as both an input and an outputto allow data to be transferred between the system and external devicessuch as a color scanner (not shown). Example interfaces include a modem,a network card such as an Ethernet card, a communications port, a PCMCIAslot and card, etc. Software and data transferred via the communicationsinterface are in the form of signals. Such signal may be any ofelectronic, electromagnetic, optical, or other forms of signals capableof being received by the communications interface. These signals areprovided to the communications interface via channel 626 which carriessuch signals and may be implemented using wire, cable, fiber optic,phone line, cellular link, RF, memory, or other means known in the arts.

The methods described can be implemented on a special purpose computer,a micro-processor or micro-controller, an ASIC or other integratedcircuit, a DSP, an electronic circuit such as a discrete elementcircuit, a programmable device such as a PLD, PLA, FPGA, PAL, PDA, andthe like. In general, any device capable of implementing a finite statemachine, that is in turn capable of implementing one or more elements ofthe flow diagrams provided herewith, or portions thereof, can be used.The teachings hereof can be implemented in hardware or software usingany known or later developed systems, structures, devices, and/orsoftware by those skilled in the applicable art without undueexperimentation from the functional description provided herein with ageneral knowledge of the relevant arts.

One or more aspects of the methods described herein are intended to beincorporated in an article of manufacture, including one or morecomputer program products, having computer usable or machine readablemedia. For purposes hereof, a computer usable or machine readable mediais, for example, a floppy disk, a hard-drive, memory, CD-ROM, DVD, tape,cassette, or other digital or analog media, or the like, which iscapable of having embodied thereon a computer readable program, one ormore logical instructions, or other machine executable codes or commandsthat implement and facilitate the function, capability, andmethodologies described herein. The article of manufacture may beincluded as part of a xerographic system, an operating system, aplug-in, or may be shipped, sold, leased, or otherwise providedseparately, either alone or as part of an add-on, update, upgrade, orproduct suite.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Variouspresently unforeseen or unanticipated alternatives, modifications,variations, or improvements therein may become apparent and/orsubsequently made by those skilled in the art, which are also intendedto be encompassed by the following claims. Accordingly, the embodimentsset forth above are considered to be illustrative and not limiting.Various changes to the above-described embodiments may be made withoutdeparting from the spirit and scope of the invention.

1. The method for calibration of a marking device's output color asmeasured at the solid area level, the method comprising: selecting acolor separation of an n-separation marking device for testing, saidmarking device having at least one adjustable actuator which regulatesan amount of colorant deposited on a substrate at the solid area level,said actuator having been previously set to a predetermined firstoperating point; selecting a target color parameter vector Tat the solidarea level comprising a point in color parameter space; printing a solidarea patch of said target color parameter vector T on a substrate usingsaid marking device with said actuator set to said first operating pointand measuring said printed patch to obtain a nominal measured colorparameter vector N; determining an amount of an offset based upon saidtarget color parameter vector T, said nominal measured color parametervector N, and a fleet gradient {right arrow over (G)}_(F); and applyingsaid determined amount of offset to said actuator setting such that saidactuator is adjusted to a second operating point.
 2. The method of claim1, wherein said offset is determined in response to a difference betweensaid nominal measured color parameter vector N and said target colorparameter vector T being outside a predetermined threshold.
 3. Themethod of claim 1, wherein said offset comprises:$x = {\frac{{\overset{->}{G}}_{F \cdot} \cdot \left( {T - N} \right)}{\left( {{\overset{->}{G}}_{F} \cdot {\overset{->}{G}}_{F}} \right)}.}$4. The method of claim 1, wherein having adjusted said actuator to saidsecond operating point, further comprising: printing a solid area patchof said target color parameter vector T on a substrate using saidmarking device and measuring said printed patch to obtain a measuredcolor parameter vector M; comparing said measured color parameter vectorM to said target color parameter vector T; and in response to a resultof said comparison being outside a predetermined threshold, adjustingsaid actuator to a third operating point.
 5. The method of claim 4,further comprising determining a new gradient {right arrow over (G)}₂wherein said new gradient comprises:${{\overset{->}{G}}_{2} = \frac{\left( {M - N} \right)}{z}},$ where z isa difference between said first and second operating points.
 6. Themethod of claim 5, further comprising determining an amount ofadjustment to be applied to said actuator such that said actuator isadjusted to a third actuator operating point, said adjustment amountcomprising:$y = \frac{{\overset{->}{G}}_{2 \cdot} \cdot \left( {T - M} \right)}{\left( {{\overset{->}{G}}_{2} \cdot {\overset{->}{G}}_{2}} \right)}$7. The method of claim 1, wherein said target color parameter vector Tis measured using a reflectance sensing device.
 8. A system forcalibration of a marking device's output color as measured at the solidarea level, the system comprising: a memory; a non-transitory storagemedium; and a processor in communication with the storage medium and thememory, the processor executing machine readable program instructionsfor performing the method of: selecting a color separation of ann-separation marking device for testing, said marking device having atleast one adjustable actuator which regulates an amount of colorantdeposited on a substrate at the solid area level, said actuator havingbeen previously set to a predetermined first operating point; selectinga target color parameter vector T at the solid area level comprising apoint in color parameter space; printing a solid area patch of saidtarget color parameter vector Ton a substrate using said marking devicewith said actuator set to said first operating point and measuring saidprinted patch to obtain a nominal measured color parameter vector N;determining an amount of an offset based upon said target colorparameter vector T, said nominal measured color parameter vector N, anda fleet gradient {right arrow over (G)}_(F); and applying saiddetermined amount of offset to said actuator setting such that saidactuator is adjusted to a second operating point.
 9. The system of claim8, wherein said offset is determined in response to a difference betweensaid nominal measured color parameter vector N and said target colorparameter vector T being outside a predetermined threshold.
 10. Thesystem of claim 8, wherein said offset comprises:$x = {\frac{{\overset{->}{G}}_{F \cdot} \cdot \left( {T - N} \right)}{\left( {{\overset{->}{G}}_{F} \cdot {\overset{->}{G}}_{F}} \right)}.}$11. The system of claim 8, wherein having adjusted said actuator to saidsecond operating point, further comprising: printing a solid area patchof said target color parameter vector T on a substrate using saidmarking device and measuring said printed patch to obtain a measuredcolor parameter vector M; comparing said measured color parameter vectorM to said target color parameter vector T; and in response to a resultof said comparison being outside a predetermined threshold, adjustingsaid actuator to a third operating point.
 12. The system of claim 11,further comprising determining a new gradient {right arrow over (G)}₂wherein said new gradient comprises:${{\overset{->}{G}}_{2} = \frac{\left( {M - N} \right)}{z}},$ where z isa difference between said first and second operating points.
 13. Thesystem of claim 12, further comprising determining an amount ofadjustment to be applied to said actuator such that said actuator isadjusted to a third actuator operating point, said adjustment amountcomprising:$y = \frac{{\overset{->}{G}}_{2 \cdot} \cdot \left( {T - M} \right)}{\left( {{\overset{->}{G}}_{2} \cdot {\overset{->}{G}}_{2}} \right)}$14. The system of claim 8, wherein said target color parameter vector Tis measured using a reflectance sensing device.
 15. A computerimplemented method for calibration of a marking device's output color asmeasured at the solid area level, the method comprising: selecting acolor separation of an n-separation marking device for testing, saidmarking device having at least one adjustable actuator which regulatesan amount of colorant deposited on a substrate at the solid area level,said actuator having been previously set to a predetermined firstoperating point; selecting a target color parameter vector Tat the solidarea level comprising a point in color parameter space; printing a solidarea patch of said target color parameter vector T on a substrate usingsaid marking device with said actuator set to said first operating pointand measuring said printed patch to obtain a nominal measured colorparameter vector N; determining an amount of an offset based upon saidtarget color parameter vector T, said nominal measured color parametervector N, and a fleet gradient {right arrow over (G)}_(F), said offsetcomprising:${x = \frac{{\overset{->}{G}}_{F \cdot} \cdot \left( {T - N} \right)}{\left( {{\overset{->}{G}}_{F} \cdot {\overset{->}{G}}_{F}} \right)}};$and applying said determined amount of offset to said actuator settingsuch that said actuator is adjusted to a second operating point.
 16. Thecomputer implemented method of claim 15, wherein said offset isdetermined in response to a difference between said nominal measuredcolor parameter vector N and said target color parameter vector T beingoutside a predetermined threshold.
 17. The computer implemented methodof claim 15, wherein having adjusted said actuator to said secondoperating point, further comprising: printing a solid area patch of saidtarget color parameter vector T on a substrate using said marking deviceand measuring said printed patch to obtain a measured color parametervector M; comparing said measured color parameter vector M to saidtarget color parameter vector T; and in response to a result of saidcomparison being outside a predetermined threshold, adjusting saidactuator to a third operating point.
 18. The computer implemented methodof claim 17, further comprising determining a new gradient {right arrowover (G)}₂ wherein said new gradient comprises:${{\overset{->}{G}}_{2} = \frac{\left( {M - N} \right)}{z}},$ where z isa difference between said first and second operating points.
 19. Thecomputer implemented method of claim 18, further comprising determiningan amount of adjustment to be applied to said actuator such that saidactuator is adjusted to a third actuator operating point, saidadjustment amount comprising:$y = \frac{{\overset{->}{G}}_{2 \cdot} \cdot \left( {T - M} \right)}{\left( {{\overset{->}{G}}_{2} \cdot {\overset{->}{G}}_{2}} \right)}$20. The computer implemented method of claim 15, wherein said targetcolor parameter vector Tis measured using a reflectance sensing device.